dcollins
Explorer
gfunk said:"If your item gives a bonus beyond the limit allowed in the Dungeon Master's Guide, multipy the portion of the market price derived from the characteristic by 10."
The maximum skill bonus granted by the DMG is +30 (Ring of Jump). Therefore, you can safely +350 skill items are beyond the limits of the DMG's (and of course, T&B, which simply clarifies and expands the DMG rules) item creation rules.
Arguably, that example is insufficient, granted that the DMG explicitly says "Skill check bonuses do not have a maximum." (p. 176, under "Bonuses from Magic")... unless the ELH has something a lot more specific.
As usual, this is one of those balance problems that are most easily fixed by DMs not allowing any arbitrary item to be made taking advantage of the DMG formulas. By default, I simply don't allow anything not in the core rulebooks to be crafted, and I expect that that's how the WOTC people playtested them.